Duct liners

ABSTRACT

Ducts are formed from rigid or semi-rigid insulation boards by forming a plurality of longitudinally extending, spaced apart, parallel kerfs in a first major surface of each of the insulation boards which is adapted to be the interior surface of the duct. When the insulation board is formed into a duct of the preselected dimensions, the shoulders of the kerfs adjoining the interior surface of the duct abut and, preferably, the insulation material in the shoulders of the kerfs is compressed. The flexibility of the insulation boards, in the direction of the widths of the insulation boards, is controlled by increasing the depths and/or the widths of the kerfs and/or decreasing the spacing between the kerfs to make the insulation board more flexible and by decreasing the depths and/or the widths of the kerfs and/or increasing the spacing between the kerfs to make the insulation board less flexible.

This application is a division of application Ser. No. 08/528,448, filedSep. 14, 1995.

BACKGROUND OF THE INVENTION

The present invention is directed to ducts and duct liners and to amethod of forming ducts and duct liners from rigid and semi-rigidinsulation boards wherein the configuration of the kerfs formed in theinsulation boards as well as the depths, widths and spacing of the kerfsin the insulation boards, control the flexibility of the insulationboards and the structural compression characteristics of the kerfsidewalls or shoulders to facilitate the formation of ducts and ductliners of desired diameters from the insulation boards.

Heating, ventilating and air conditioning systems, exhaust systems andother similar gas flow systems found in large building structures, suchas, but not limited to convention centers, sports domes, warehouses,factories and the like require large diameter, high capacity, insulatedround or flat oval air ducts for conveying conditioned air from airconditioning units to locations throughout the building structure andfor returning air to the air conditioning units for heating or coolingand recirculation or for conveying other gases such as exhaust gases.These large diameter, insulated round or flat oval air ducts typicallyrange from about twenty inches to over one hundred inches in diameter.

Presently, the most commonly used ducts for such large, high capacity,air and gas flow systems are double wall insulated metal ducts. Theselarge diameter, high capacity, metal ducts for conveying air and othergases comprise an outer tubular metal shell, an inner perforated tubularmetal shell and one or more layers of glass fiber insulation blankets orother insulation materials intermediate the outer tubular shell and theinner tubular shell. These ducts normally have either a round or a flatoval transverse cross section. Due to their construction which, inessence, comprises a first tubular metal duct contained within a secondtubular metal duct, and the labor required to both assemble the firstmetal duct within the second metal duct and insulate the space betweenthe ducts while maintaining the ducts concentric with respect to eachother, these double wall, insulated metal ducts for conveying gases,such as conditioned air, return air, exhaust gases and the like, arerelatively expensive.

A second duct, currently in use for conveying gases, comprises a round,tubular metal shell internally lined with a round tubular, preformed,self-supporting, molded, glass fiber insulation sold by SchullerInternational Inc., under the trademark "SPIRACOUSTIC". This duct systemexhibits good thermal and acoustical insulating properties andeliminates the need for a costly perforated metal liner. However,because of manufacturing restrictions relating to the molding of theglass fiber insulation into its tubular shape, this duct system has beensize restricted for use with tubular metal shells having internaldiameters of twenty-six inches or less.

Thus, although the molded glass fiber insulation liners provide a costeffective lining system with good performance characteristics fortubular metal ducts having internal diameters of twenty-six inches orless, there has been a need to provide large diameter air and gashandling ducts with an insulated lining system other than the costlyperforated metal lining system discussed above.

SUMMARY OF THE INVENTION

The ducts and duct liners of the present invention comprise rigid orsemi-rigid insulation boards (commonly referred to as "duct boards")which have a plurality of parallel kerfs (narrow grooves) in their gasstream surfaces that enable the insulation boards to be curved or bentabout an axis parallel to the kerfs into tubular shapes to form ducts orduct liners. Typically, the insulation boards are made of a fibrousinsulation, a cellular foam insulation or a fibrous and cellular foaminsulation laminate. The insulation boards are typically four feet byten feet or eight feet by ten feet and from about one inch to about fourinches thick. For the purposes of this application the dimension of theinsulation board parallel to the kerfs is defined as the length of theinsulation board and the dimension of the insulation board perpendicularto the kerfs is defined as the width of the insulation boards.

Where the duct or duct liner being formed has a circumference equal toor less than the width of a kerfed insulation board, the insulationboard or a portion thereof is bent and formed into a tube of the desireddiameter with the abutting lateral edges of the insulation boardpreferably being taped or otherwise held together. As a duct liner, theduct is then inserted into a tubular metal shell having internaldimensions equal to or slightly less than the external dimensions of theduct liner. Where the duct or duct liner being formed has acircumference greater than the width of a kerfed insulation board, theinsulation board is taped or otherwise affixed to a second insulationboard or portion thereof and the joined insulation boards are bent andformed into a tube of the desired diameter which can be inserted into atubular metal shell.

The gas stream surfaces of the insulation boards may be treated oruntreated. However, with respect to fibrous insulation boards,preferably, the gas stream surfaces of the insulation boards areprovided with a coating, such as a polymeric coating, or are providedwith a facing to encapsulate fibers and/or particles of the insulationboards within the insulation boards.

The kerfs formed in the gas stream surfaces of the insulation boards, toenable the insulation boards to be bent into a tubular shape, are sizedso that the shoulders of the kerfs adjoining the air stream surfaces ofthe insulation boards close together and are subjected to compressionwhen the insulation boards are bent into a tubular shape of the desireddimensions for the duct or duct liner being fabricated. With theinsulation material of the insulation boards in compression at theshoulders of the kerfs, the insulation boards retain the tubular shapeinto which the insulation boards have been formed. If the kerfs are towide, the shoulders of the kerfs adjoining the gas stream surfaces ofthe insulation boards are not subjected to compression when theinsulation boards are bent and shaped into a tubular configuration ofthe desired dimensions and the duct or duct liners formed from theinsulation boards do not retain the desired tubular shape. Furthermore,since the kerfs formed in the insulation panels of the ducts or ductliners of the present invention close together at the gas stream surfaceof the ducts or duct liners, fibers or dust in the kerf sidewalls arenot exposed to the interior of the ducts or duct liners and the air orother gas streams being conveyed by the ducts.

In one preferred embodiment of the present invention, the narrow kerfshave straight sidewalls extending perpendicular to the gas streamsurfaces of the insulation boards with shoulders adjoining the gasstream surface which close when the insulation boards are bent andcurved into the desired tubular shape. While the sidewalls of thesekerfs meet at the gas stream surfaces of the insulation boards to placethe shoulders of the kerfs adjoining the gas stream surface of the ductin compression and to seal off the kerfs, a series of air spaces, havingtriangular cross sections, are formed in the walls of the ducts or ductliners formed from the insulation boards. These air spaces reduce theinsulating properties of the ducts or duct liners formed from theinsulation boards and when using kerfs with straight sidewalls whichextend perpendicular to the gas stream surfaces of the insulationboards, the sizes of these air spaces increase as the thicknesses of theinsulation boards used to form the ducts or duct liners are increased.

Vee-shaped kerfs or substantially vee-shaped kerfs can eliminate orsubstantially eliminate the air spaces within the walls of the ductsformed from the kerfed insulation boards of the present invention and,to assure optimum insulating characteristics in the ducts or duct linersformed from the kerfed insulation boards, vee-shaped kerfs orsubstantially vee-shaped kerfs should be used in insulation boards twoinches thick or greater. However, if the vee-shaped kerfs are too wideat the gas stream surfaces of the insulation boards, the shoulders ofthe kerfs adjoining the gas stream surfaces of the insulation boards donot close together and are not subjected to compression when theinsulation boards are formed into a tubular shape. As discussed above,the insulation material in the shoulders of the kerfs and preferably,extending for the entire or substantially the entire depth of thesidewalls, should be subjected to compression so that the insulationboards will retain the tubular shape into which the insulation boardshave been formed and the shoulders should be closed together so thatfibers and/or particles in the kerf sidewall surfaces are not exposed tothe interior of the ducts and the high velocity gas streams beingconveyed by the ducts. Thus, the kerfs formed in the gas stream surfacesof the insulation boards in accordance with a preferred embodiment ofthe present invention have vee-shaped or substantially vee-shaped crosssections with the widths of the kerfs at the gas stream surfaces of theinsulation boards being controlled so that the air spaces in the wallsof the ducts or duct liners formed from the insulation boards areeliminated or substantially eliminated and the shoulders of the kerfsadjoining the gas stream surfaces of the insulation boards and,preferably, the entire or substantially the entire remaining portions ofthe kerf sidewalls are placed under compression when the insulationboards are formed into the duct or duct liner of preselected dimensions.

The flexibility of the insulation boards, in the direction of the widthsof the insulation boards, may be controlled by the depth, width and/orspacing of the kerfs formed in the gas stream surface of the insulationboards. An insulation board with kerfs of a certain depth and width thatare spaced from each other a certain spacing will exhibit a certainflexibility. To make the insulation boards more flexible, the depths ofthe kerfs can be increased, the widths of the kerfs can be increasedand/or the spacing between kerfs can be decreased. To make theinsulation boards less flexible, the depths of the kerfs can bedecreased, the widths of the kerfs can be decreased and/or the spacingbetween the kerfs can be increased.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an insulation board of the present invention with a kerfed gasstream surface.

FIG. 2 is a duct or duct liner of the present invention, with a roundtransverse cross section, formed from a kerfed insulation board.

FIG. 3 is a duct of the present invention comprising a tubular metalshell, with a round transverse cross section, lined with a duct linerformed from a kerfed insulation board.

FIG. 4 is a duct or duct liner of the present invention, with a flatoval transverse cross section, formed from a kerfed insulation board.

FIG. 5 is a duct of the present invention comprising a tubular metalshell, with a flat oval transverse cross section, lined with a ductliner formed from a kerfed insulation board.

FIG. 6 is a schematic fragmentary section through an insulation boardshowing a kerf with straight sidewalls and a rectangular cross section.

FIG. 6A is a schematic fragmentary section through the insulation boardof FIG. 6 showing the kerf after the insulation board has been bent toform a tubular shape.

FIG. 7 is a schematic fragmentary section through an insulation boardshowing a kerf with stepped sidewalls to approximate a vee-shaped crosssection.

FIG. 7A is a schematic fragmentary section through the insulation boardof FIG. 7 showing the kerf after the insulation board has been bent toform a tubular shape.

FIG. 8 is a schematic fragmentary section through an insulation boardshowing a kerf with a vee-shaped transverse cross section.

FIG. 8A is a schematic fragmentary section through the insulation boardof FIG. 8 showing the kerf after the insulation board has been bent toform a tubular shape.

FIG. 9 is a schematic fragmentary section through an insulation boardshowing a kerf with a substantially vee-shaped transverse cross section.

FIG. 9A is a schematic fragmentary section through the insulation boardof FIG. 9 showing the kerf after the insulation board has been bent toform a tubular shape.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 shows an insulation board 20 having a first major surface 22which is a gas stream or interior surface and a second major surface 24which is an outside surface. The insulation board 20 may be a rigid orsemi-rigid insulation board comprising a fibrous insulation material,such as a glass fiber insulation, a cellular foam material, such as apolyimide foam insulation, or a composite board of fibrous insulationand cellular foam insulation.

In certain industrial applications, such as the handling of exhaustgases where coated or faced gas stream surfaces are not required on theduct or duct liner formed from the insulation board and/or where theduct or duct liner formed from the insulation board is to be frequentlyreplaced, the gas stream surface 22 of the insulation board may beuntreated as an additional cost savings. However, preferably, the gasstream surface has a coating or facing 26 covering the gas streamsurface, such as but not limited to, a polymeric coating, anon-combustible foil facing, a synthetic polymer film, a metallic foilcomposite or a treated, non-woven mat (e.g. a polyester mat coated witha polymeric coating). For insulation boards 20 that are going to beformed into ducts or duct liners for heating, ventilating and airconditioning systems, a polymeric coating on the gas stream surface ofthe insulation board can add to the performance characteristics of theduct system and permit regular maintenance and cleaning or the ducts orduct liners. In addition, the polymeric coating can be applied to thegas stream surface before or after the kerfing operation. The use of anon-combustible foil, a synthetic polymer film, a metallic foilcomposite or a treated non-woven mat as a facer on the gas streamsurface of the insulation board is advantageous when the insulationboards are going to be formed into ducts or duct liners for certainindustrial and commercial operations and also may offer certain costsavings over the polymeric coating.

The outside surface 24 of the insulation board 20 is preferably,provided with a moisture barrier facing 28, such as but not limited to afoil-scrim-kraft facing. The use of a moisture barrier facing preventsthe buildup of moisture between the outer surface of a duct liner formedfrom the insulation board and an outer metal shell.

The insulation boards 20 are typically about ten feet long by four feetwide or eight feet wide and range in thickness from about one inch toabout four inches. The glass fiber insulation boards typically used inthe present invention include a phenolic binder and have a densityranging from about 2.4 to about 4.5 lbs/cu.ft. Cellular foam insulationboards typically used in the present invention have a density rangingfrom about 0.5 to about 2.0 lbs/cu.ft.

As shown in FIG. 1, the gas stream surface 22 of the insulation board 20is provided with a plurality of kerfs 30. The kerfs 30 extend the lengthof the insulation board, are spaced with respect to each other andextend parallel with respect to each other. The kerfing of theinsulation board 20 forms a plurality of longitudinally extendingsegments 32 which are hinged together by the hinge portions 34 of theinsulation boards intermediate the bottoms of the kerfs 30 and theoutside surfaces 24 of the insulation boards plus the facings 28 on theoutside surfaces of the insulation boards. Preferably, the kerfs 30 areformed by a plurality of appropriately spaced and driven steel, ceramicor carbide saw blades.

When the insulation boards 20 are to be formed into ducts or duct linersranging in diameter from about sixteen inches to about thirty inches,the spacing between the kerfs 30 is preferably /2 inches or 1.57 incheson centers. When the insulation boards 20 are to be formed into ducts orduct liners ranging in diameter from about thirty inches to one hundredinches or more, the spacing between the kerfs is preferably or 2 incheson centers. The kerfing of the insulation board 20 allows the insulationboard to be bent or curved in the direction of its width to enable theinsulation board to be formed into the tubular shape of a duct or ductliner. The kerfs 30 can not be placed too close together or theinsulation material of the insulation board in the longitudinal segments32 will lose its integrity. If the kerfs 30 are spaced too far apart,the insulation board 20 will not have the flexibility in the directionof its width to enable the insulation board to be bent and curved into atubular shape of the desired dimensions.

The kerfs 30 are narrow and typically have a width at the gas streamsurface of the insulation board of about 3/32 of an inch for insulationboards one and one half inches in thickness or less. The depths of thekerfs 30 preferably range from about 70% to about 90% of the thicknessof the insulation boards 20. Thus, the hinge portions 34 of the kerfedinsulation boards intermediate the longitudinal segments 32 aretypically about one quarter to one half of an inch thick.

As shown in FIGS. 2 and 4, after the insulation board 20 has been formedinto a duct 36 or 38 respectively, the lateral edges of the insulationboard are taped or otherwise secured together so that the insulationboard will remain in its round or flat oval tubular configuration. Theduct may then be inserted into a metal shell 40 or 42 to form a ductliner for the metal shell and form a round or flat oval insulated metalduct 44 or 46, as shown in FIGS. 3 and 5. As discussed above, if theinternal circumference of the metal shell being lined is greater thanthe width of the insulation board 20, a portion of another insulationboard can be secured to the insulation board to obtain the needed width.

The outer tubular metal shells 40 and 42 used in the present inventionare typically conventional tubular shells commonly used for industrialand commercial applications. The round or flat oval outer tubular shellsare typically made of spirally wound sheet metal strips with adjacentconvolutions of the metal strips joined by a conventional spiral seam.The insulated metal ducts may be joined together by male/femaleconnectors; sleeves; outer flanges, such as the flanges 48 and 50 shownin FIGS. 3 and 5; or other conventional means.

FIGS. 6 through 9A schematically represent various transverse crosssectional shapes which may be used for the kerfs 30 formed in theinsulation boards 20 of the present invention. FIGS. 6, 7, 8, and 9represent the transverse cross sections of the kerfs 30 as formed in theinsulation board 20 prior to forming the insulation board into a tubularconfiguration to form a duct or duct liner 36 or 38. FIGS. 6A, 7A, 8Aand 9A represent the transverse cross sections of the kerfs 30 after theinsulation board 20 has been bent or curved into a tubular configurationto form a duct or duct liner 36 or 38 of preselected transversedimensions.

As shown in FIGS. 6 and 6A, the kerf 30a has a narrow rectangular orsubstantially rectangular transverse cross section with sidewalls 52 and54 which extend perpendicular or substantially perpendicular to the gasstream surface 22 of the insulation board 20 and parallel orsubstantially parallel to a plane 56 extending perpendicular to the gasstream surface 22 of the insulation board and bisecting the kerf 30aalong the longitudinal centerline of the kerf. The kerf 30a also hasshoulders 57 adjoining the gas stream surface 22 of the insulation board20. When the insulation board 20 is bent or formed into a tubularconfiguration to form a duct or duct liner as shown in FIG. 6A, theshoulders 57 of the kerf 30a, adjoining the gas stream surface 22 of theinsulation board 20, come together, but there is a triangular air space58 formed in the wall of duct or duct liner. Preferably, the shoulders57 of the kerf 30a not only abut when the insulation board is bent intoa desired tubular configuration of preselected transverse dimensions,but the shoulders are pressed together, when the insulation board isformed into the desired tubular configuration of the preselectedtransverse dimensions, with sufficient force to compress the insulationmaterial in the shoulders 57 of the kerf 30a so that the tubular shapeof the duct or duct liner formed from the insulation board 20 isretained after the insulation board has been formed into a duct or ductliner.

As shown in FIGS. 7 and 7A, the narrow kerf 30b has an upper portion 60and a lower portion 62 which are both rectangular or substantiallyrectangular in transverse cross section with the lower portion 62 of thekerf 30b being narrower than the upper portion 60. The stepped divergingsidewalls 64 and 66 of the kerf 30b each comprise upper and lowerportions which extend perpendicular or substantially perpendicular tothe gas stream surface 22 of the insulation board and parallel orsubstantially parallel to a plane 68 extending perpendicular to the gasstream surface 22 of the insulation board and bisecting the kerf 30balong a longitudinal centerline of the kerf. The upper and lowerportions of each of the sidewalls are joined by an intermediate sectionwhich extends at an angle to the plane 68. The kerf 30b also hasshoulders 69 adjoining the gas stream surface 22 of the insulation board20. When the insulation board 20 is bent or formed into a tubularconfiguration to form a duct or duct liner as shown in FIG. 7A, theshoulders 57 of the kerf 30b, adjoining the gas stream surface 22 of theinsulation board, come together. While there are two small substantiallytriangular air spaces 70 and 72 formed in the wall of the duct or ductliner, the combined volume of the spaces 70 and 72 is less than thevolume 58 formed by using a kerf having the cross sectional shape ofkerf 30a.

Preferably, the kerf 30b are formed so that the shoulders 69 of the kerf30b not only abut when the insulation board 20 is bent into a desiredtubular configuration of preselected transverse dimensions, but theshoulders are pressed together, when the insulation board is formed intothe desired tubular configuration of the preselected transversedimensions, with sufficient force to compress the insulation material inthe shoulders 69 of the kerf 30b so that the tubular shape of the ductor duct liner formed from the insulation board 20 is retained after theinsulation board has been formed into a duct or duct liner. Mostpreferably, the kerf 30b is formed so that the insulation material inthe sidewalls of the kerf 30b, for the entire depth or substantially theentire depth of the kerf, is subjected to compression when theinsulation board 20 is bent into the desired tubular configuration ofthe preselected transverse dimensions to eliminate or substantiallyeliminate the air spaces 70 and 72 and to further rigidify the duct orduct liner formed from the insulation board 20.

As shown in FIGS. 8 and 8A, the kerf 30c has a narrow vee-shapedtransverse cross section with sidewalls 74 and 76 which meet at the baseor bottom of the kerf 30c and extend to the gas stream surface of theinsulation board 20. The sidewalls diverge with respect to each otherand a plane 78, extending perpendicular to the gas stream surface 22 ofthe insulation board 20 and bisecting the kerf 30c along thelongitudinal centerline of the kerf 30c, from the base of the kerf 30cto the gas stream surface 22 of the insulation board 20. The kerf 30chas shoulders 79 adjoining the gas stream surface 22 of the insulationboard 20. When the insulation board 20 is bent or formed into a tubularconfiguration to form a duct or duct liner as shown in FIG. 8A, theshoulders 79 of the kerf 30c, adjoining the gas stream surface 22 of theinsulation board 20, come together and the sidewalls abut along theirsurfaces so that no air space is formed in the wall of the duct or ductliner.

Preferably, the kerf 30c is formed so that the shoulders 79 of the kerf30c not only abut when the insulation board is bent into a desiredtubular configuration of preselected transverse dimensions, but theshoulders are pressed together, when the insulation board has beenformed into the desired tubular configuration of the preselectedtransverse dimensions, with sufficient force to compress the insulationmaterial in the shoulders 79 of the kerf 79 so that the tubular shape ofthe duct or duct liner formed from the insulation board 20 is retainedafter the insulation board has been formed into a duct or duct liner.Most preferably, the kerf 30c is formed so that the insulation materialin the sidewalls of the kerf 30c, for the entire or substantially theentire depth of the kerf, is subjected to compression when theinsulation board is bent into the tubular configuration of thepreselected transverse dimensions to further rigidify the duct or ductliner formed from the insulation board 20.

As shown in FIGS. 9 and 9A, the kerf 30d has a narrow vee-shapedtransverse cross section with sidewalls 80 and 82 which extend from abase or bottom of the kerf 30d to the gas stream surface 22 of theinsulation board 20. The kerf 30d is like kerf 30c except, unlike kerf30c, the sidewalls 80 and 82 of kerf 30d do not meet at the base of thekerf. Instead the sidewalls 80 and 82 are narrowly spaced from eachother at the base of the kerf 30d and diverge with respect to each otherand a plane 84, extending perpendicular to the gas stream surface 22 ofthe insulation board 20 and bisecting the kerf 30d along thelongitudinal centerline of the kerf 30d, from the base of the kerf 30dto the gas stream surface 22 of the insulation board 20.

The kerf 30d has shoulders 85 adjoining the gas stream surface 22 of theinsulation board. When the insulation board 20 is bent or formed into atubular configuration to form a duct or duct liner as shown in FIG. 9A,the shoulders 85 of the kerf 30d, adjoining the gas stream surface 22 ofthe insulation board 20, come together and a very narrow air space 86 isformed in the wall of the duct or duct liner at the bottom of the kerf30d.

Preferably, the kerf 30d is formed so that the shoulders 85 of the kerf30d not only abut when the insulation board 20 is bent into a desiredtubular configuration of preselected transverse dimensions, but theshoulders are pressed together, when the insulation board is formed intothe desired tubular configuration of the preselected transversedimensions, with sufficient force to compress the insulation material inthe shoulders 85 of the kerf 30d so that the tubular shape of the ductor duct liner formed from the insulation board 20 is retained after theinsulation board has been formed into a duct or duct liner. Mostpreferably, the kerf 30d is formed so that the insulation material inthe sidewalls of the kerf 30d, for substantially the entire depth of thekerf, is placed under compression when the insulation board is formedinto the tubular configuration of the preselected transverse dimensionsto substantially eliminate the air space 86 and further rigidify theduct or duct liner formed from the insulation board 20.

The widths of the kerf openings at the gas stream surface 22 of theinsulation boards 20 must be controlled. If the longitudinally extendingopenings of the kerfs 30 are too wide at the gas stream surface of theinsulation board 20, the openings may not close and the shoulders or theshoulders and other portions of the kerf sidewalls may not becompressed, as preferred, when the insulation board is formed into atubular configuration of selected dimensions to form a duct or ductliner. The widths of the openings of the kerfs 30 of the presentinvention may be defined by a pair of planes 90 and 92 which extend froma common vertex point 94, located at the second major surface 24 ofinsulation board in the plane bisecting the kerf 30, through thejunctures of the kerf sidewalls with the gas stream surface 22, as shownin FIGS. 6, 7, 8 and 9. The included angle X between these two planesranges from about 3° to about 10° depending on the thickness of theinsulation board and the diameter of the duct or duct liner being formedfrom the kerfed insulation board 20. The greater the thickness of theinsulation board 20 being kerfed and the greater the diameter of theduct or duct liner being formed from the insulation board, the smallerthe included angle X between the planes 90 and 92.

The flexibility of the insulation boards 20, in the direction of thewidths of the insulation boards, may be controlled by the depths and/orthe widths of and/or spacing between the kerfs 30 in the gas streamsurface 22 of the insulation boards. An insulation board 20 with kerfsof a certain depth and width that are spaced from each other a certainspacing will exhibit a certain flexibility. To make the insulationboards more flexible, the depths of the kerfs 30 can be increased, thewidths of the kerfs can be increased and/or the spacing between thekerfs 30 can be decreased. To make the insulation boards less flexible,the depths of the kerfs 30 can be decreased, the widths of the kerfs canbe decreased and/or the spacing between the kerfs 30 can be increased.In describing the invention, certain embodiments have been used toillustrate the invention and the practices thereof. However, theinvention is not limited to these specific embodiments as otherembodiments and modifications within the spirit of the invention willreadily occur to those skilled in the art on reading this specification.Thus, the invention is not intended to be limited to the specificembodiments disclosed, but is to be limited only by the claims appendedhereto.

What is claimed is:
 1. A kerfed insulation board for forming a duct of apreselected transverse dimensions and cross section comprising:aninsulation board having a first major surface adapted to be an interiorsurface of a duct formed from said insulation board and a second majorsurface adapted to be an outer surface of a duct formed from saidinsulation board; said insulation board having a length adapted toextend parallel to a longitudinal centerline of a duct formed from saidinsulation board, a width and a thickness; kerfs in said first majorsurface of said insulation board for making said insulation boardflexible in a direction of said width of said insulation board andfoldable into a tubular configuration of preselected transversedimensions and cross section; each of said kerfs having a depth; each ofsaid kerfs extending parallel with respect to and being spaced fromother of said kerfs; each of said kerfs extending said length of saidinsulation board; each of said kerfs having sidewalls which extend froma kerf base portion adjacent but spaced inwardly from said second majorsurface of said insulation board to said first major surface of saidinsulation board; each of said kerfs having shoulder portions adjoiningsaid first major surface of said insulation board; and each of saidkerfs having a width at said first major surface of said insulationboard; said width of said kerfs at said first major surface of saidinsulation board pressing said shoulder portions of said kerfs intoabutment with said insulation in said shoulder portions being incompression, when said insulation board is formed into a duct of saidpreselected transverse dimensions and cross section, to retain saidinsulation board in said preselected transverse cross section with noexposure of the kerf sidewalls at the interior surface of the duct andwith any loose fibers or dust from said kerf sidewalls being confinedwithin said kerfs.
 2. The kerfed insulation board according to claim 1,wherein: said sidewalls of each of said kerfs diverge with respect toeach other from said kerf base portion to said first major surface ofsaid insulation board.
 3. The kerfed insulation board according to claim1, wherein: each of said kerfs is formed with a generally vee-shapedtransverse cross section.
 4. The kerfed insulation board according toclaim 1, wherein: said thickness of said insulation board is betweenabout one inch and four inches and said base portion of each of saidkerfs is spaced from said second major surface of said insulation boardbetween about one quarter and about one half of an inch.
 5. A ductcomprising:an insulation board having a first major surface forming aninterior surface of a duct and a second major surface forming an outersurface of said duct; said insulation board having a length extendingparallel to a longitudinal centerline of said duct, a width and athickness; kerfs in said first major surface of said insulation board;each of said kerfs having a depth; each of said kerfs extending parallelwith respect to and being spaced from other of said kerfs; each of saidkerfs extending said length of said insulation board; said insulationboard being folded into a tubular configuration of preselectedtransverse dimensions and cross section; and each of said kerfs havingshoulders adjoining said first major surface which abut each other withinsulation material in said shoulder portions being in compression toretain said insulation board in said preselected transverse crosssection with no exposure of the kerf sidewalls at the interior surf aceof the duct and with any loose fibers or dust from said kerf sidewallsconfined within said kerfs.
 6. The duct according to claim 5, wherein:said thickness of said insulation board is between about one inch andabout four inches and said kerfs have a base portion spaced from saidsecond major surface of said insulation board between about one quarterand about one half of an inch.
 7. A kerfed insulation board for forminga duct of a preselected transverse dimensions and cross sectioncomprising:an insulation board having a first major surface adapted tobe an interior surface of a duct formed from said insulation board and asecond major surface adapted to be an outer surface of a duct formedfrom said insulation board; said insulation board having a lengthadapted to extend parallel to a longitudinal centerline of a duct formedfrom said insulation board, a width and a thickness; and kerfs in saidfirst major surface of said insulation board for making said insulationboard flexible in a direction of said width of said insulation board andfoldable into a tubular configuration of preselected transversedimensions and cross section; each of said kerfs having a depth; each ofsaid kerfs extending parallel with respect to and being spaced fromother of said kerfs; each of said kerfs extending said length of saidinsulation board; each of said kerfs having sidewalls which extend froma kerf base portion adjacent but spaced inwardly from said second majorsurface of said insulation board to said first major surface of saidinsulation board; each of said kerfs having a stepped transverse crosssection with said sidewalls of each of said kerfs having a plurality ofportions, extending substantially perpendicular to said first majorsurface of said insulation board, which are joined by portions extendingat an angle to said first major surface of said insulation board; eachof said kerfs having shoulder portions adjoining said first majorsurface of said insulation board; and each of said kerfs having a widthat said first major surface of said insulation board such that, whensaid insulation board is formed into a duct of said preselectedtransverse dimensions and cross section, said shoulder portions of saidkerfs abut and insulation material in said shoulder portions of saidkerfs is placed in compression.
 8. A kerfed insulation board for forminga duct of a preselected transverse dimensions and cross sectioncomprising:an insulation board having a first major surface adapted tobe an interior surface of a duct formed from said insulation board and asecond major surface adapted to be an outer surface of a duct formedfrom said insulation board; said insulation board having a lengthadapted to extend parallel to a longitudinal centerline of a duct formedfrom said insulation board, a width and a thickness; said insulationboard comprising fibrous insulation having a density between about 2.4and about 4.5 pounds per cubic foot; and kerfs in said first majorsurface of said insulation board for making said insulation boardflexible in a direction of said width of said insulation board andfoldable into a tubular configuration of preselected transversedimensions and cross section; each of said kerfs having a depth; each ofsaid kerfs extending parallel with respect to and being spaced fromother of said kerfs; each of said kerfs extending said length of saidinsulation board; each of said kerfs having sidewalls which extend froma kerf base portion adjacent but spaced inwardly from said second majorsurface of said insulation board to said first major surface of saidinsulation board; each of said kerfs having shoulder portions adjoiningsaid first major surface of said insulation board; and each of saidkerfs having a width at said first major surface of said insulationboard such that, when said insulation board is formed into a duct ofsaid preselected transverse dimensions and cross section, said shoulderportions of said kerfs abut and said fibrous insulation in said shoulderportions of said kerfs is placed in compression.
 9. A kerfed insulationboard for forming a duct of a preselected transverse dimensions andcross section comprising:an insulation board having a first majorsurface adapted to be an interior surface of a duct formed from saidinsulation board and a second major surface adapted to be an outersurface of a duct formed from said insulation board; said insulationboard having a length adapted to extend parallel to a longitudinalcenterline of a duct formed from said insulation board, a width and athickness; said insulation board comprising a cellular foam insulationhaving a density between about 0.5 and about 2.0 pounds per cubic foot;and kerfs in said first major surface of said insulation board formaking said insulation board flexible in a direction of said width ofsaid insulation board and foldable into a tubular configuration ofpreselected transverse dimensions and cross section; each of said kerfshaving a depth; each of said kerfs extending parallel with respect toand being spaced from other of said kerfs; each of said kerfs extendingsaid length of said insulation board; each of said kerfs havingsidewalls which extend from a kerf base portion adjacent but spacedinwardly from said second major surface of said insulation board to saidfirst major surface of said insulation board; each of said kerfs havingshoulder portions adjoining said first major surface of said insulationboard; and each of said kerfs having a width at said first major surfaceof said insulation board such that, when said insulation board is formedinto a duct of said preselected transverse dimensions and cross section,said shoulder portions of said kerfs abut and said foam insulation insaid shoulder portions of said kerfs is placed in compression.
 10. Aduct comprising:an insulation board having a first major surface formingan interior surface of a duct and a second major surface forming anouter surface of said duct; said insulation board having a lengthextending parallel to a longitudinal centerline of said duct, a widthand a thickness; kerfs in said first major surface of said insulationboard; each of said kerfs having a depth; each of said kerfs extendingparallel with respect to and being spaced from other of said kerfs; eachof said kerfs extending said length of said insulation board; each ofsaid kerfs having shoulders adjoining said first major surface whichabut each other with insulation material in said shoulder portions beingin compression; and said duct being a liner enclosed within a tubularmetal shell.
 11. A duct comprising:an insulation board having a firstmajor surface forming an interior surface of a duct and a second majorsurface forming an outer surface of said duct; said insulation boardhaving a length extending parallel to a longitudinal centerline of saidduct, a width and a thickness; said insulation board comprising afibrous insulation having a density between about 2.4 and about 4.5pounds per cubic foot; and kerfs in said first major surface of saidinsulation board; each of said kerfs having a depth; each of said kerfsextending parallel with respect to and being spaced from other of saidkerfs; each of said kerfs extending said length of said insulationboard; each of said kerfs having shoulders adjoining said first majorsurface which abut each other with said fibrous insulation in saidshoulder portions being in compression.
 12. A duct comprising:aninsulation board having a first major surface forming an interiorsurface of a duct and a second major surface forming an outer surface ofsaid duct; said insulation board having a length extending parallel to alongitudinal centerline of said duct, a width and a thickness; saidinsulation board comprising a cellular foam insulation having a densitybetween about 0.5 and about 2.0 pounds per cubic foot; and kerfs in saidfirst major surface of said insulation board; each of said kerfs havinga depth; each of said kerfs extending parallel with respect to and beingspaced from other of said kerfs; each of said kerfs extending saidlength of said insulation board; each of said kerfs having shouldersadjoining said first major surface which abut each other with said foaminsulation in said shoulder portions being in compression.